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. general, under oxidizing conditions, pure phase uranium minerals are unlikely to occur unless unusually high concentrations of phosphate or silica are present. Uraninite (UO2) in the amorphous and crystalline forms is a common uranium mineral in both high temperature hydrothermal vein deposits and in soils in the presence of organic carbon or other reducing conditions. The presence of uraninite in the environment is a direct result of its low solubility and high stability, even under oxidizing conditions. Uraninite, once oxidized to U02.25 has been found to be very stable in the environment, even when subjected to oxidizing conditions. Solid-Solution Phases More common than pure phase uranium minerals are minerals of other elements that contain trace concentrations of uranium. When one element substitutes in a mineral for another element it is called a solid-solution. Such substitutions are most common when the charge and size of the substituting ion is similar to those which it replaces. Amorphous minerals, without a rigid crystal structure, are more acceptable to charge and size differences and can often incorporate more trace constituents than the corresponding crystalline phases. When a trace element such as uranium precipitates at the same time as another mineral and becomes incorporated into that phase, the process is referred to as co-precipitation. Uranyl ion (U02+2) is a form of U(VI) and tends to substitute for calcium into minerals such as hydroxyapatite. Other minerals such as calcite have a more rigid crystal structure which does not readily incorporate the large uranyl ion. However, U(IV) is a smaller ion that can substitute for calcium within calcite and aragonite. Some possible mineral phases which could substitute uranium as a trace constituent are presented in Table 5-4. Table 54 Minerals Into which Uranium My Substitute Mineral Formula Anhydrite CaSO4 Aragonite CaCO3 Calcite CaC03 Dolomite CaM C03 2 Gypsum CaSO4:2H20 H drox a atite Cas PO4 30H Uranium co-precipitation with calcite and aragonite is believed to be a significant process within the vadose zone at the Hanford Site (Davis, et al. 2005), while uranium co-precipitation with gypsum is common in the gypsum stacks of facilities that process uranium-containing phosphate rock. Apatite is the main uranium-containing mineral within phosphate rock and is believed to be an important control of U(VI) mobility in phosphate-containing groundwater.